Systems and processes for improved combustion control

ABSTRACT

Combustion systems and processes are disclosed. A cross-limiting override control system is configured to transmit remote set points to flow controllers configured to control the flow rates of a primary firing fuel and air to a combustion apparatus. A temperature controller is configured to transmit a temperature demand signal based on a temperature in a combustion zone in the combustion apparatus. An analyzer controller is configured to transmit a stoichiometric ratio control signal based on an excess oxygen level exiting the combustion zone in the combustion apparatus. The cross-limiting override control system includes a dynamic stoichiometric ratio multiplier element configured to receive the stoichiometric ratio control signal transmitted from the analyzer controller, a high signal select element configured to receive the temperature demand signal transmitted from the temperature controller, and a low signal select element configured to receive the temperature demand signal transmitted from the temperature controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/330,588, filed May 3, 2010, which is incorporated by reference into this specification.

TECHNICAL FIELD

This disclosure relates to fuel-fired combustion systems and processes, including utility and waste oxidizing systems and processes.

BACKGROUND OF THE INVENTION

Combustion systems and processes oxidize organic compounds, which fuel the combustion reactions. Combustion systems and processes may be used, for example, to incinerate organic waste compounds. For instance, waste oxidizing systems and processes may be used to incinerate compounds classified as volatile organic compounds and other government-regulated organic wastes. In this manner, waste oxidizing combustion systems and processes effectively dispose of regulated organic wastes by converting the organic compounds to combustion products, such as, carbon dioxide and water, which are more readily disposed of than the regulated organic compounds. Waste oxidizing combustion systems and processes may be employed, for example, in chemical manufacturing operations as part of a plant waste management system.

Combustion systems and processes may also be used to produce heat energy to do useful work. For example, utility/process boilers oxidize organic fuels, such as coal, fuel oil, propane, or natural gas, to produce heat energy used to produce steam. The steam may be used to drive turbines, pumps, or compressors. Utility/process boiler combustion systems may also be used in chemical manufacturing operations to produce steam used to provide process heating or as a chemical reactant, for example. Waste oxidizing boilers may be used to simultaneously incinerate organic waste compounds and produce steam.

SUMMARY OF THE INVENTION

In a non-limiting embodiment, a combustion system comprises a combustion apparatus, a primary fuel feed line configured to feed a primary firing fuel to the combustion apparatus, and a combustion air feed line configured to feed air to the combustion apparatus. A temperature controller is configured to transmit a temperature demand signal based on a temperature in a combustion zone in the combustion apparatus. An analyzer controller is configured to transmit a stoichiometric ratio control signal based on an excess oxygen level exiting the combustion zone in the combustion apparatus. A cross-limiting override control system is configured to transmit remote set points to flow controllers configured to control the flow rates of the primary firing fuel and the air to the combustion apparatus. The cross-limiting override control system comprises a dynamic stoichiometric ratio multiplier element configured to receive the stoichiometric ratio control signal transmitted from the analyzer controller, a high signal select element configured to receive the temperature demand signal transmitted from the temperature controller, and a low signal select element configured to receive the temperature demand signal transmitted from the temperature controller.

In another non-limiting embodiment, a fuel management control process comprises measuring a temperature in a combustion zone in a combustion apparatus, measuring an excess oxygen level exiting the combustion zone in the combustion apparatus, measuring a flow rate of a primary firing fuel to the combustion apparatus, and measuring a flow rate of air to the combustion apparatus. A temperature demand signal is transmitted to a logic control point from a temperature controller based on the temperature in the combustion zone in the combustion apparatus. A stoichiometric ratio control signal is transmitted to the logic control point from an analyzer controller based on the excess oxygen level exiting the combustion zone in the combustion apparatus. A signal representing the flow rate of the primary firing fuel to the combustion apparatus is transmitted to the logic control point. A signal representing the flow rate of the air to the combustion apparatus is transmitted to the logic control point. Remote set points are calculated in the logic control point for flow controllers configured to control the flow rates of the primary firing fuel and the air to the combustion apparatus. The remote set points are transmitted to the flow controllers.

It is understood that the invention disclosed and described in this specification is not limited to the embodiments summarized in this Summary.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and characteristics of the non-limiting and non-exhaustive embodiments disclosed and described in this specification may be better understood by reference to the accompanying figures, in which:

FIG. 1 is a schematic process diagram illustrating a combustion system and process under the control of a cross-limiting override fuel management control system;

FIG. 2 is a schematic process diagram illustrating a combustion system and process under the control of a cross-limiting override fuel management control system;

FIG. 3 is a schematic process diagram illustrating a waste oxidizing combustion system and process under the control of a cross-limiting override fuel management control system;

FIG. 4 is a schematic process diagram illustrating a waste oxidizing combustion apparatus under the control of a cross-limiting override fuel management control system;

FIG. 5 is a schematic process diagram illustrating the control logic of a cross-limiting override fuel management control system;

FIG. 6 is a schematic process diagram illustrating a waste oxidizing combustion system and process under the control of a cross-limiting override fuel management control system with dynamic stoichiometric ratio control;

FIG. 7 is a schematic process diagram illustrating a waste oxidizing combustion apparatus under the control of a cross-limiting override fuel management control system with dynamic stoichiometric ratio control;

FIG. 8 is a schematic process diagram illustrating the control logic of a cross-limiting override fuel management control system with dynamic stoichiometric ratio control;

FIG. 9 is a schematic process diagram illustrating a two-stage waste oxidizing combustion apparatus under the control of a cross-limiting override fuel management control system with dynamic stoichiometric ratio control;

FIG. 10 is a schematic process diagram illustrating a two-stage waste oxidizing boiler under the control of a cross-limiting override fuel management control system with dynamic stoichiometric ratio control; and

FIG. 11 is graph showing a reduction in natural gas usage per liquid waste fuel flow rate in a waste oxidizing boiler comprising a cross-limiting override fuel management control system with dynamic stoichiometric ratio control.

The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of various non-limiting and non-exhaustive embodiments according to the present disclosure.

DESCRIPTION OF THE INVENTION

Various embodiments are described and illustrated in this specification to provide an overall understanding of the structure, function, operation, and use of the disclosed systems and processes. It is understood that the various embodiments described and illustrated in this specification are non-limiting and non-exhaustive. Thus, the invention is not limited by the description of the various non-limiting and non-exhaustive embodiments disclosed in this specification. Rather, the invention is defined solely by the claims. The features and characteristics illustrated and/or described in connection with various embodiments may be combined with the features and characteristics of other embodiments. Such modifications and variations are intended to be included within the scope of this specification. As such, the claims may be amended to recite any features or characteristics expressly or inherently described in, or otherwise expressly or inherently supported by, this specification. Further, Applicant reserves the right to amend the claims to affirmatively disclaim features or characteristics that may be present in the prior art. Therefore, any such amendments comply with the requirements of 35 U.S.C. §112, first paragraph, and 35 U.S.C. §132(a). The various embodiments disclosed and described in this specification can comprise, consist of, or consist essentially of the features and characteristics as variously described herein.

Reference throughout this specification to “various non-limiting embodiments,” or the like, means that a particular feature or characteristic may be included in an embodiment. Thus, use of the phrase “in various non-limiting embodiments,” or the like, in this specification does not necessarily refer to a common embodiment, and may refer to different embodiments. Further, the particular features or characteristics may be combined in any suitable manner in one or more embodiments. Thus, the particular features or characteristics illustrated or described in connection with various embodiments may be combined, in whole or in part, with the features or characteristics of one or more other embodiments without limitation. Such modifications and variations are intended to be included within the scope of the present specification.

The grammatical articles “one”, “a”, “an”, and “the”, as used in this specification, are intended to include “at least one” or “one or more”, unless otherwise explicitly indicated. Thus, the articles are used in this specification to refer to one or more than one (i.e., to “at least one”) of the grammatical objects of the article. By way of example, “a component” means one or more components, and thus, possibly, more than one component is contemplated and may be employed or used in an implementation of the described embodiments. Further, the use of a singular noun includes the plural, and the use of a plural noun includes the singular, unless the context of the usage requires otherwise.

Ratio control is used to maintain the flow rate of one stream in a system of process at a specified proportion relative to the flow rate of a different stream in the system or process. Ratio control may be used in combustion systems and processes, such as, for example, fired heaters, boilers, furnaces, oxidizers, and similar fuel-burning unit operations, to maintain specified ratios of oxidizer-to-fuel. For instance, hydrocarbon fuel-fired process equipment may use ratio control to maintain specified combustion air-to-fuel gas ratios in order to achieve complete combustion of the fuel to produce heat, carbon dioxide, and water, and to minimize the formation of carbon monoxide, unburned fuel, and nitrogen oxides (NO_(x)).

In combustion systems and processes, the air-to-fuel ratio may be expressed on a mass basis. Air may be provided to the combustion zone in a combustion apparatus at a mass flow rate that is stoichiometrically matched to the mass flow rate of fuel delivered to the burner in the combustion apparatus in accordance with the following generic equation for fuel combustion reactions:

fuel+air→useful heat+CO₂+H₂O+CO+unburned fuel+NO_(x)+waste heat

The oxygen (O₂) needed to burn fuel may be provided from combustion air fed to a combustion apparatus. If the air-to-fuel ratio is too low, there is insufficient oxygen present to completely convert fuel to carbon dioxide and water. This condition may be referred to as “fuel-rich” or “air-lean” operation. Fuel-rich operation leads to incomplete combustion of the fuel. As the level of oxygen decreases below the stoichiometric equivalent needed for complete combustion, carbon monoxide production increases. As the air-to-fuel ratio decreases further, partially burned and unburned fuel levels increase. This is particularly undesirable because carbon monoxide and un-combusted hydrocarbon fuels are considered pollutants that are regulated by various governments. Incomplete combustion also increases the amount of fuel wastage as fuel that does not completely burn decreases the amount of useful heat energy produced in a combustion system or process.

If the air-to-fuel ratio is too high, excess oxygen and increased nitrogen are fed to the combustion zone. This condition may be referred to as “fuel-lean” or “air-rich” combustion. The excess oxygen and increased nitrogen entering the combustion zone absorb and carry away a portion of the heat energy produced by the combustion reactions, which decreases the temperature of the combustion flame and the gases in the combustion zone. This decrease in operating temperature decreases the amount of heat energy that may be extracted from the combustion reactions to do useful work. The increased nitrogen levels present in the flame may also increase the formation of NO_(x), which is also a government-regulated emission.

The relationships between the air-to-fuel ratio, pollution formation, and heat energy losses provide a basis for control system design. The minimum amount of air required under ideal conditions to just complete the conversion of a hydrocarbon fuel to carbon dioxide and water is called “theoretical” or “stoichiometric” air. The stoichiometric amount of air is determined from a balanced chemical reaction equation summarizing the oxidation combustion reactions. For example, 1 mole of methane (the major component of natural gas) requires 2 moles of oxygen for complete combustion:

CH₄+2O₂→CO₂+2H₂O

However, real combustion processes involve a complex set of several hundred to several thousand chemical reactions that occur simultaneously, particularly in waste oxidizing systems used to incinerate variable waste streams containing multiple organic compounds. Further, real combustion systems and processes have imperfect mixing of fuel and air, and flow velocities limit residence time in the combustion zone of combustion equipment. Therefore, under real conditions, incomplete combustion will result if air is fed to a combustion apparatus in the exact theoretical or stoichiometric proportion to the fuel. As a result, combustions systems are generally run, at least in part, under slightly fuel-lean conditions in order to ensure complete combustion.

For a particular combustion system or process design, there may be a target fuel-lean air-to-fuel ratio that balances the competing effects so that carbon monoxide formation, NO_(x) formation, and heat losses are minimized, while ensuring complete combustion of the fuel. For example, combustion systems firing natural gas, propane, or fuel oil as the primary firing fuel may operate with excess air in the range of 105% to 130% of stoichiometric air. This means that the combustion air supplied to the combustion zone has 1.05 to 1.30 times the amount of the stoichiometric oxygen required for complete combustion of the fuel under ideal conditions.

In various non-limiting embodiments, in order to maintain target air-to-fuel ratios, combustion systems and processes may be operated under a fuel management control system employing cross-limiting override control. Fuel management control automates the modulation of the primary fuel flow rate and the combustion air flow rate supplied to a combustion apparatus using a combination of high and low signal select elements and a ratio multiplier element. Signal select elements receive two input signals and forward one of the two input signals as an output signal. A low signal select element passes the input signal having a lower numerical value. A high signal select element passes the input signal having a higher numerical value. Signal select elements, therefore, enable decision-making logic to be included in an air-to-fuel ratio control system. A ratio multiplier element receives an input signal, multiplies the input signal by a ratio value, and forwards the product as an output signal. Signal select and ratio multiplier elements may be implemented, for example, using electrical circuit hardware or as function blocks in programmable logic controllers (PLC) or distributed control systems (DCS).

A non-limiting example of a fuel management control system for a combustion process is illustrated in FIG. 1. A primary air feed line 11 and a primary fuel feed line 12 supply combustion air and primary firing fuel to a combustion apparatus 10. The flow rates of air and fuel to the combustion apparatus 10 are controlled, at least in part, by valves 13 and 14 in feed lines 11 and 12, respectively. The flow rate through the air feed line 11 is measured by a flow transmitter 15 and the flow rate through the fuel feed line 12 is measured by a flow transmitter 16. A signal representing the combustion air flow rate through the air feed line 11 is transmitted to a low signal select element 22. A signal representing the fuel flow rate through the fuel feed line 12 is transmitted to a stoichiometric ratio multiplier element 20. The fuel flow rate signal is multiplied by a stoichiometric ratio value in the multiplier element 20 and the product is transmitted as an output signal to a high signal select element 21.

In various non-limiting embodiments, the stoichiometric ratio value may be an operator-defined manual input that establishes the amount of oxygen (supplied in the air feed line 11) required for complete combustion of the fuel (supplied in the fuel feed line 12). For example, combustion systems may be controlled to operate with excess air in the range of 105% to 130% of stoichiometric air, in which case the stoichiometric ratio may range from 1.05 to 1.30. The stoichiometric ratio manual input value may be determined, for example, using stoichiometric analysis of the oxidation chemistry when using well-characterized constant fuels, or empirically from historical data when using variable fuels such as organic waste streams.

A signal representing the product of a fuel flow rate signal and a stoichiometric ratio value is referred to herein as a “stoichiometrically-scaled fuel signal.” Referring to FIG. 1, a stoichiometrically-scaled fuel signal is transmitted from the stoichiometric ratio multiplier element 20 to the high signal select element 21. The stoichiometrically-scaled fuel signal is the product of the fuel flow signal from the flow transmitter 16 and the operator-defined stoichiometric ratio.

A firing demand signal is transmitted to the high signal select element 21 and the low signal select element 22. The firing demand signal enters the low signal select element 22 as a candidate signal for a remote set point (RSP) for a fuel flow controller 18, which controls the action of valve 14 in fuel line 12. The signal representing the air flow rate through the air feed line 11 is the other candidate signal for the RSP for the fuel flow controller 18. The firing demand signal enters the high signal select element 21 as a candidate signal for a RSP for an air flow controller 17, which controls the action of valve 13 in air line 11. The stoichiometrically-scaled fuel signal is the other candidate signal for the RSP for the air flow controller 17.

The high signal select element 21 transmits the firing demand signal or the stoichiometrically-scaled fuel signal, whichever has a higher numerical value, as the RSP for the air flow controller 17. The low signal select element 22 transmits the firing demand signal or the signal representing the air flow rate through the air feed line 11, whichever has a lower numerical value, as the RSP for the fuel flow controller 18.

If the firing demand signal is greater than the stoichiometrically-scaled fuel signal, the high signal select element 21 will transmit the firing demand signal through as the RSP for the air flow controller 17, which will cause the valve 13 to increase air flow to the combustion apparatus 10. The low signal select element 22 will not transmit the firing demand increase signal because it is greater than the signal representing the air flow rate through the air feed line 11. Therefore, the RSP for the fuel flow controller 18 will track the increasing air flow rate as it increases in response to the increased firing demand signal, maintaining tighter control over the air-to-fuel ratio fed to the combustion apparatus 10.

If the firing demand signal is less than the signal representing the air flow rate through the air feed line 11, the low signal select element 22 will transmit the firing demand signal through as the RSP for the fuel flow controller 18, which will cause the valve 14 to decrease fuel flow to the combustion apparatus 10. The high signal select element 21 will not transmit the firing demand decrease signal because it is less than the stoichiometrically-scaled fuel signal. Therefore the RSP for the air flow controller 17 will track the decreasing fuel flow rate as it decreases in response to the decreased firing demand signal, maintaining tighter control over the air-to-fuel ratio fed to the combustion apparatus 10.

When firing demand increases, the high signal select element 21 ensures that the combustion air flow fed to the combustion apparatus 10 increases to meet demand, and the low signal select element 22 operates as an override to ensure that the lagging fuel flow process variable follows the leading combustion air flow process variable to control the air-to-fuel ratio fed to the combustion apparatus 10. When firing demand decreases, the low signal select element 22 ensures that the fuel flow fed to the combustion apparatus 10 decreases to meet demand, and the high signal select element 21 operates as an override to ensure that the lagging air flow process variable follows the leading fuel flow process variable to control the air-to-fuel ratio fed to the combustion apparatus 10.

A fuel management control system employing cross-limiting override control, as illustrated in FIG. 1, ensures that a combustion apparatus is temporarily supplied with extra air during operational changes that disturb a controlled target air-to-fuel ratio until the target air-to-fuel ratio is restored. The temporary operation under fuel-lean conditions temporarily increases the amount of lost heat energy due to heating of the excess air, but avoids the emission of carbon monoxide and unburned fuel. Cross-limiting override control also prevents an increased firing demand from causing increases in both air and fuel flows to a combustion apparatus, which prevents run-away combustion and reduces fire hazards. Cross-limiting override control also ensures that there is always enough oxygen being fed to a combustion apparatus to maintain flame quality, which increases reliability in systems configured to trip and automatically shutdown on a flameout.

In various non-limiting embodiments, a firing demand signal may be based on one or more downstream process variables. For example, a firing demand signal may be based on the pressure in a steam header that is fed from a combustion boiler under cross-limiting override fuel management control. The firing demand signal may be computed by a plant master controller and/or a boiler master controller to maintain a target steam pressure in the steam header. A firing demand signal for fuel management control may also be based, for example, on at least one of the temperature in a combustion zone of a combustion apparatus, a duct temperature downstream of the combustion zone, and a temperature of a heat transfer fluid exiting a heat exchanger downstream of a combustion apparatus.

In various non-limiting embodiments, a firing demand signal is based on the combustion zone temperature in a combustion apparatus. A non-limiting example of a cross-limiting override fuel management control system for a combustion system and process is illustrated in FIG. 2, in which the firing demand signal is based on a combustion zone temperature. The temperature in the combustion zone of a combustion apparatus 10 is measured by a temperature transmitter (not shown), which is part of a temperature indicating controller 25. The temperature indicating controller 25 transmits a firing demand output signal based on a comparison of the measured combustion zone temperature to a target temperature set point. If the measured temperature is less than the temperature set point, the temperature indicating controller 25 transmits a firing/temperature demand increase signal. If the measured temperature is greater than the temperature set point, the temperature indicating controller 25 transmits a firing/temperature demand decrease signal.

The firing/temperature demand signal is transmitted from the temperature indicating controller 25 to the high signal select element 21 and the low signal select element 22. If the temperature demand signal is greater than the stoichiometrically-scaled fuel signal, the high signal select element 21 will transmit the temperature demand increase signal through as the RSP for the air flow controller 17, which will cause the valve 13 to increase air flow to the combustion apparatus 10. The low signal select element 22 will not transmit the temperature demand increase signal because it is greater than the signal representing the air flow rate through the air feed line 11. Therefore, the RSP for the fuel flow controller 18 will track the increasing air flow rate as it increases in response to the temperature demand increase signal, maintaining tighter control over the air-to-fuel ratio fed to the combustion apparatus 10.

If the temperature demand signal is less than the signal representing the air flow rate through the air feed line 11, the low signal select element 22 will transmit the temperature demand decrease signal through as the RSP for the fuel flow controller 18, which will cause the valve 14 to decrease fuel flow to the combustion apparatus 10. The high signal select element 21 will not transmit the temperature demand decrease signal because it is less than the stoichiometrically-scaled fuel signal. Therefore the RSP for the air flow controller 17 will track the decreasing fuel flow rate as it decreases in response to the temperature demand decrease signal, maintaining tighter control over the air-to-fuel ratio fed to the combustion apparatus 10.

When temperature demand increases, the high signal select element 21 ensures that the combustion air flow fed to the combustion apparatus 10 increases to meet the demand for increased combustion temperature, and the low signal select element 22 operates as an override to ensure that the lagging fuel flow process variable follows the leading combustion air flow process variable to control the air-to-fuel ratio fed to the combustion apparatus 10. When temperature demand decreases, the low signal select element 22 ensures that the fuel flow fed to the combustion apparatus 10 decreases to meet the demand for decreased combustion temperature, and the high signal select element 21 operates as an override to ensure that the lagging air flow process variable follows the leading fuel flow process variable to control the air-to-fuel ratio fed to the combustion apparatus 10.

In various non-limiting embodiments, a waste oxidizing combustion system or process may comprise a cross-limiting override fuel management control system. For example, FIG. 3 illustrates a waste oxidizing combustion system comprising a cross-limiting override fuel management control system, which operates as described above in connection with FIG. 2. In addition to the primary air feed line 11 and primary fuel feed line 12, a waste fuel line 30 feeds waste fuels to the combustion apparatus 10. The waste fuels fed to the combustion apparatus 10 may include, for example, organic liquids and off-gases produced as by-products of upstream chemical manufacturing unit operations. The flow rate of waste fuel to the combustion apparatus 10 is measured by a flow transmitter 31. A signal representing the waste fuel flow rate and the signal representing the primary firing fuel flow rate are transmitted to a summation element 35, where a signal representing total fuels is calculated.

In various embodiments, both gas-phase and liquid-phase waste fuels may be oxidized in a single waste oxidizing combustion apparatus.

For example, gas-phase waste fuel may be fed to a waste oxidizing combustion apparatus directly into the firebox downstream of the burner. Gas-phase waste fuel may be fed into the firebox through a separate injection point or through a ring inlet around the primary combustion air inlet. Liquid-phase waste fuel may be fed to a waste oxidizing combustion apparatus at the burner with atomizing steam to aid in distribution and mixing of the liquid waste fuel. Accordingly, waste oxidizing combustion equipment may be used to oxidize organic waste fluids from upstream chemical manufacturing unit operations in addition to the combustion of the primary firing fuel.

Air-to-fuel ratio control is particularly important for waste oxidizing combustion systems and processes because a trip that automatically shuts down the waste oxidizing combustion equipment may require an immediate shutdown of the upstream process equipment to prevent the accumulation and/or emission of regulated waste materials in reportable quantities. Shutdown of upstream process equipment also increases manufacturing down time, which decreases plant efficiency and increases lost profits.

The control of a waste oxidizing combustion system is more complex than a typical utility combustion system because of the varying stoichiometry and heat content of the waste fuels fed to waste oxidizing combustion equipment, whereas typical utility combustion systems burn primary firing fuels having a generally constant composition. The mixing of primary fuel, waste fuel, and combustion air is important for flame stability in waste oxidizing combustion systems. Generally, waste oxidizing combustion systems are configured to trip and automatically shutdown if the combustion flame becomes unstable or goes out. Therefore, flame stability is a prerequisite for the overall reliability of the combustion system, and flame stability is directly related to control of the air-to-fuel ratio of the system.

Due to the multiple dynamic variables of liquid and/or gas waste fuels, many waste oxidizing combustion systems run in a local automatic or manual control mode. A manual control mode requires that operator personnel monitor all the process variables at all times, which significantly increases the likelihood of human error. A local automatic control mode includes static set points for the combustion air feed flow and primary fuel feed flow process variables. Nevertheless, operators still need to monitor the combustion zone and change the static set points for the combustion air feed flow and primary fuel feed flow in order to maintain controlled combustion as indicated by flame quality, for example. The variability in the waste fuel flow rates, composition, and heat content, if not effectively controlled for, may result in upset conditions that trip and automatically shut down the combustion system. Therefore, waste oxidizing combustion systems and processes may comprise a cross-limiting override fuel management control system to compensate, at least in part, for waste fuel variability.

Referring to FIG. 4, a waste oxidizing system 50 comprises a waste oxidizing apparatus 60. The waste oxidizing apparatus 60 comprises a burner 65 and a firebox 66. A primary air feed line 51 and a primary fuel feed line 52 supply combustion air and primary combustion fuel to the combustion apparatus 60 at the burner 65. The flow rates of air and primary firing fuel to the combustion apparatus 60 are controlled, at least in part, by valves 53 and 54 in feed lines 51 and 52, respectively. The flow rate through the air feed line 51 is measured by flow transmitter 55 and the flow rate through the primary fuel feed line 52 is measured by flow transmitter 56. A signal representing the combustion air flow rate through the air feed line 51 is transmitted to a logic control point (LCP) 80. A signal representing the fuel flow rate through fuel feed line 52 is transmitted to the LCP 80.

A gas waste fuel feed line 61 feeds gas-phase waste fuels to the combustion apparatus 60 through a separate injection point in the firebox 66. A liquid waste fuel feed line 63 feeds liquid-phase waste fuels to the combustion apparatus 60 at the burner 65. The flow rates of the gas-phase waste fuels and the liquid-phase waste fuels fed to the combustion apparatus 60 are measured by flow transmitters 62 and 64, respectively. Signals representing the flow rates of the gas-phase waste fuels and the liquid-phase waste fuels fed to the combustion apparatus are transmitted to the LCP 80. The temperature in the combustion zone in the firebox 66 of the combustion apparatus 60 is measured by a temperature transmitter (not shown), which is part of a temperature indicating controller 70. The temperature indicating controller 70 transmits a temperature demand output signal to the LCP 80 based on a comparison of the measured combustion zone temperature to a target temperature set point, as described above in connection with FIG. 2.

The control logic for the LCP 80 is shown in greater detail in FIG. 5. A signal representing the combustion air flow rate is transmitted from flow transmitter 55 to a low signal select element 82. (For ease of illustration, the flow transmitter 55 and the air flow controller 57 in FIG. 4 are schematically illustrated in FIG. 5 as a common air flow indicating controller element 57′). The signals representing the flow rates of the primary fuel, the gas-phase waste fuels, and the liquid-phase waste fuels are summed using stoichiometric calculations in a summation element 84 to provide a feed forward signal representing the total fuel input to the combustion system. In various embodiments, because the composition of the waste fuels may be variable, the stoichiometric calculations performed in summation element 84 may incorporate estimates of the waste fuel compositions based on known information about the upstream unit operations producing the waste. The stoichiometric calculation for total fuels in summation element 84 accounts for all of the oxidizable feed material to ensure that enough air is supplied to the waste oxidizing apparatus for complete combustion. The signal representing the total fuel input to the combustion system is transmitted to a stoichiometric ratio multiplier element 90. The total fuel signal is multiplied by a stoichiometric ratio value in the multiplier element 90 and the product, which is a stoichiometrically-scaled fuel signal, is transmitted as an output signal to a high signal select element 81.

The temperature indicating controller 70 transmits a temperature demand output signal based on a comparison of the measured combustion zone temperature to a target temperature set point. If the measured temperature is less than the temperature set point, the temperature indicating controller 70 transmits a temperature demand increase signal. If the measured temperature is greater than the temperature set point, the temperature indicating controller 70 transmits a temperature demand decrease signal. The temperature demand signal is transmitted from the temperature indicating controller 70 to the high signal select element 81 and the low signal select element 82.

If a temperature demand increase signal is greater than the stoichiometrically-scaled fuel signal, the high signal select element 81 will transmit the temperature demand increase signal through as the RSP for the air flow controller 57, which will cause the valve 53 to increase air flow to the combustion apparatus 60. The low signal select element 82 will not transmit the temperature demand increase signal because it is greater than the signal representing the air flow rate through the air feed line 51. Therefore, the RSP for the fuel flow controller 58 will track the increasing air flow rate as it increases in response to the temperature demand increase signal, maintaining tighter control over the air-to-fuel ratio fed to the combustion apparatus 60. (For ease of illustration, the flow transmitter 56 and the fuel flow controller 58 in FIG. 4 are schematically illustrated in FIG. 5 as a common air flow indicating controller element 58′).

If a temperature demand decrease signal is less than the signal representing the air flow rate through the air feed line 51, the low signal select element 82 will transmit the temperature demand decrease signal through as the RSP for the fuel flow controller 58, which will cause the valve 54 to decrease fuel flow to the combustion apparatus 60. The high signal select element 81 will not transmit the temperature demand decrease signal because it is not greater than the stoichiometrically-scaled fuel signal. Therefore the RSP for the air flow controller 57 will track the decreasing fuel flow rate as it decreases in response to the temperature demand decrease signal, maintaining tighter control over the air-to-fuel ratio fed to the combustion apparatus 60.

In various embodiments, the stoichiometric ratio value used to multiply the total fuel signal may be a dynamic value based on the amount of oxygen exiting the combustion zone in a combustion apparatus. For example, FIG. 6 illustrates a waste oxidizing combustion system comprising a cross-limiting override fuel management control system, as described above in connection with FIGS. 2 and 3, and further comprising an analyzer indicating controller 40 that measures the excess oxygen exiting the combustion zone and transmits a control signal to a stoichiometric ratio multiplier element 20′ to control a dynamic stoichiometric ratio value.

Referring to FIG. 7, a waste oxidizing system 50′ (similar to the waste oxidizing system 50 illustrated in FIG. 4) comprises an analyzer indicating controller 75 that measures the excess oxygen exiting the combustion zone in the firebox 66 of the combustion apparatus 60, and transmits a control signal to the LCP 80′. The control logic for the LCP 80′ is shown in greater detail in FIG. 8, which operates in like manner as the LCP 80 described in connection with FIG. 5. However, rather than a static stoichiometric ratio value, as used in stoichiometric ratio multiplier element 90 in FIG. 5, a dynamic stoichiometric ratio value is used in stoichiometric ratio multiplier element 90′ in FIG. 8, which is based on the controller output of the analyzer indicating controller 75.

The oxygen analyzer indicating controller 75 transmits a stoichiometric control signal based on a comparison of the measured oxygen level to a target oxygen level set point. If the measured oxygen content is less than the target set point, the analyzer indicating controller 75 transmits a controller output signal for a stoichiometric ratio increase. If the measured oxygen content is greater than the target set point, the analyzer indicating controller 75 transmits a controller output signal for a stoichiometric ratio decrease. A dynamic stoichiometric ratio based on excess oxygen levels in combination with cross-limiting override control automatically compensates for any changes in composition and heat content of the waste fuels fed to a waste oxidizing combustion system.

For example, an increase in the heat content of a waste fuel fed to a waste oxidizing combustion system will result in a decrease in the excess oxygen level exiting the combustion zone because the higher heat content of the waste fuel requires more oxygen to complete the combustion reactions. An increase in the stoichiometric ratio in response to the decrease in oxygen level automatically compensates for the increased heat content of the waste fuel by increasing the combustion air flow, which is controlled using a cross-limiting override control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements.

Likewise, a decrease in the heat content of a waste fuel fed to a waste oxidizing combustion system will result in an increase in the excess oxygen level exiting the combustion zone because the lower heat content of the waste fuel requires less oxygen to complete the combustion reactions. A decrease in the stoichiometric ratio in response to the increase in oxygen level automatically compensates for the decreased heat content of the waste fuel by decreasing the combustion air flow, which is controlled using a cross-limiting override control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements.

In various embodiments, a cross-limiting override fuel management control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements may be used to control the air-to-fuel ratio to a multi-stage waste oxidizing system. FIG. 9 illustrates a two-stage waste oxidizing system 150. A primary fuel feed line, a liquid waste fuel feed line, and a primary combustion air feed line provide primary fuel, liquid waste fuel, and primary combustion air to the combustion apparatus at the burner. A gas waste fuel line provides gas waste fuel to the combustion apparatus downstream from the burner between the primary combustion air inlet and a secondary combustion air inlet. A secondary combustion air feed line provides secondary combustion air to the combustion apparatus downstream from the gas waste fuel line inlet.

The relative flow rates of the primary combustion air and secondary combustion air fed to the combustion apparatus establishes two combustion zones: a reduction zone and an oxidation zone. The reduction zone runs under fuel-rich conditions during normal operation by controlling the primary air flow at a sub-stoichiometric level. For example, the reduction zone may be run at 60% to 85% stoichiometric air, which results in incomplete combustion reactions and the formation of carbon monoxide and partially oxidized fuels, but minimizes NO formation because the fuel-rich conditions produce lower combustion temperatures with little to no excess oxygen to react with nitrogen. The sub-stoichiometric ratio target for the reduction zone may be determined in accordance with the specifications for the combustion equipment. The oxidation zone runs under fuel-lean conditions during normal operation by controlling the secondary air flow. For example, the oxidation zone may be run at 105% to 130% stoichiometric air. This ensures that the carbon monoxide and partially oxidized fuels produced in the reduction zone are completely oxidized to carbon dioxide and water in the oxidation zone.

Multi-stage combustion systems and processes have reduced carbon monoxide, partially oxidized hydrocarbon, and NO_(x) emissions relative to single-stage combustion systems and processes. However, it is more difficult to maintain flame stability and target air-to-fuel ratios during upset conditions in multi-stage combustion systems and processes. When the reduction zone is operating under fuel-rich conditions, adding more fuel causes the reduction zone temperature to decrease and the oxidation zone temperature to increase. When the oxidation zone temperature increases under these conditions, manual and local automatic control systems often increase the fuel flow rate to compensate, which further decreases the air-to-fuel ratio in the reduction zone. This adjustment may destabilize the flame, which may pulse because there is not enough oxygen in the reduction zone to sustain combustion. Under these conditions, the flame appears to shrink and grow in pulses while burning in the oxidation zone but not in the reduction zone. This problem effects the lifetime of the equipment by having the flame reach areas not protected by refractory and may often cause a trip and automatic shutdown of the combustion system due to a perceived flameout at the burner.

A cross-limiting override control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements mitigates these issues and improves the flame stability and control of the air-to-fuel ratio in multi-stage combustion systems. Referring to FIG. 9, a cross-limiting override fuel management control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements is illustrated in connection with a two-stage waste oxidizing system 150.

The primary control points for the fuel management system are the oxidizing zone temperature and the excess oxygen level exiting the oxidizing zone. The temperature in the oxidizing zone is measured by a temperature transmitter (not shown), which is part of a temperature indicating controller 170. The temperature indicating controller 170 transmits a temperature demand output signal to a LCP 180 based on a comparison of the measured combustion zone temperature to a target temperature set point, as described above in connection with FIGS. 2 and 4. An oxygen analyzer indicating controller 175 measures the excess oxygen exiting the oxidation zone and transmits a control signal to the LCP 180 based on a comparison of the measured oxygen level to a target oxygen level set point, as described in connection with FIG. 7. The LCP 180 operates in like manner as the LCP 80′, which is described above in connection with FIG. 8.

As illustrated in FIG. 9, the output signals of the LCP 180 provide RSPs to flow indicating controllers on the primary air feed line and the primary fuel feed line. Separate flow indicating controllers are present on the waste fuel lines; however the set points for these controllers are not dynamically determined by the fuel management control system comprising the dynamic stoichiometric ratio multiplier element and high and low signal select elements. The temperature indicating controller 170 also runs an automatic control loop for a hot condensate off stream to quickly reduce the oxidizing zone temperature if the temperature approaches a high temperature trip condition. The oxygen analyzer indicating controller 175 also runs an automatic control loop for the secondary air feed line. The loop output of this oxygen analyzer loop is transmitted to the LCP and controls the stoichiometric ratio.

In various embodiments, a dynamic stoichiometric ratio, which is controlled by the loop output of an oxygen analyzer controller measuring excess oxygen exiting the combustion zone of a single-stage combustion system or the final oxidizing zone of a multi-stage combustion system, is limited to certain minimum and maximum ratios. These minimum and maximum stoichiometric ratio limits may be determined, for example, based on historical operating data and/or field testing of a combustion system. For example, for a waste oxidizing combustion system, the dynamic stoichiometric ratio controlled by the loop output of an oxygen analyzer controller may be limited to 0.5 to 2.0 (i.e., 50% to 200% of stoichiometric air). In this manner, the dynamic stoichiometric ratio automatically compensates for variable heat content of waste fuels, but is prevented from driving either primary fuel flow or combustion air flow to very high or very low values, which could cause unstable conditions.

Likewise, limits may be placed on the output signal of the flow controller in the secondary air feed lines in a multi-stage combustion system. For example, if the valve on a secondary air feed line goes full open; the decreased pressure drop could cause a significant loss of primary air flow to the reduction zone, which could cause flame pulsing as described above. Therefore, limits may be placed on a secondary air feed line valve controlled by an automatic loop separate from the fuel management control system. These minimum and maximum valve limits may be determined, for example, based on historical operating data and/or field testing of a combustion system.

In various embodiments, a cross-limiting override fuel management control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements may be used to control the air-to-fuel ratio of any fuel-burning unit operation, such as, for example, a waste oxidizing boiler. Waste oxidizing boilers may be used to simultaneously incinerate organic waste compounds and produce steam. For example, FIG. 10 illustrates a two-stage waste oxidizing boiler comprising a cross-limiting override fuel management control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements. The cross-limiting override fuel management system illustrated in FIG. 10 operates in like manner as the systems described above.

The systems and processes described herein may be implemented using any fuel-fired combustion system, such as, for example, a natural gas-fired waste oxidizing boiler. Other applicable combustion systems include, for example, waste oxidizing furnaces and utility boilers fired with primary hydrocarbon fuels, such as, for example, natural gas, propane, fuel oil, and the like. The control sub-systems and sub-processes described herein may be implemented using any combination of hardware and/or software. The instruments and function blocks illustrated and described in this specification may be implemented by any suitable indicator, transmitter, and/or controller equipment, which may be hardware-based, software-based, or a combination of hardware and software. For instance, the summation elements, ratio multiplier elements, and signal select elements may be implemented, for example, using electrical circuit hardware or as function blocks in programmable logic controllers (PLC) or distributed control systems (DCS).

It is to be understood that the descriptions and illustrations of the embodiments disclosed in this specification have been simplified to illustrate only those features and characteristics that are relevant to a clear understanding of the disclosed embodiments, while eliminating, for purposes of clarity, other features and characteristics. Persons having ordinary skill in the art, upon considering this specification, will recognize that other features and characteristics may be desirable in a particular implementation or application of the disclosed embodiments. For example, the process diagrams provided in this specification do not show function blocks that match control loop process gains or function blocks that compensate for non-linear sensor operation. However, because such other features and characteristics may be readily ascertained and implemented by persons having ordinary skill in the art upon considering this specification, and are, therefore, not necessary for a complete understanding of the disclosed embodiments, a description of such features, characteristics, and the like, is not provided in this specification.

The non-limiting and non-exhaustive examples that follow are intended to further describe various non-limiting and non-exhaustive embodiments without restricting the scope of the embodiments described in this specification.

EXAMPLES Example 1

A natural gas-fired single-stage waste oxidizing boiler was modified to incorporate a cross-limiting override fuel management control system comprising a dynamic stoichiometric ratio multiplier element and high and low signal select elements. The waste oxidizing boiler used local automatic control loops with static set points before modification. The modified waste oxidizing boiler was similar to the waste oxidizing boiler illustrated in FIG. 10. The modification resulted in significant reductions iri natural gas usage per liquid waste fuel oxidation rates as shown in FIG. 11.

The cross-limiting override fuel management control system with dynamic stoichiometric ratio control significantly decreased the amount of natural gas primary fuel needed to operate the waste oxidizing boiler at the same average operating temperature. The modification resulted in an average 27% savings in natural gas costs during a year of operation, which corresponded to a primary fuel cost savings of $2 MM for the year at $7/MMBtu. In addition, the modification caused a 23% overall energy savings, a reduction in NO_(x) emissions of 32%, and a reduction in CO₂ emissions of 22%. Further, there have not been any reported trips due to improper fuel management of the boiler since implementation.

Example 2

Two two-stage waste oxidizing incinerators were modified to incorporate cross-limiting override fuel management control systems comprising dynamic stoichiometric ratio multiplier elements and high and low signal select elements. The waste oxidizing incinerators used local automatic control loops with static set points before modification. The modifications resulted in significant reductions in natural gas usage per waste oxidation rates as shown in FIGS. 12 and 13.

The systems and process disclosed herein provide improved control schemes that increase the efficiency and reliability of combustion devices while also reducing primary fuel usage and regulated emissions. The systems and process may have particular utility for combustion devices that oxidize and incinerate waste fuels of unknown or variable heat content.

This specification has been written with reference to various non-limiting and non-exhaustive embodiments. However, it will be recognized by persons having ordinary skill in the art that various substitutions, modifications, or combinations of any of the disclosed embodiments (or portions thereof) may be made within the scope of this specification. Thus, it is contemplated and understood that this specification supports additional embodiments not expressly set forth herein. Such embodiments may be obtained, for example, by combining, modifying, or reorganizing any of the disclosed steps, components, elements, features, aspects, characteristics, limitations, and the like, of the various non-limiting embodiments described in this specification. In this manner, Applicant(s) reserve the right to amend the claims during prosecution to add features as variously described in this specification, and such amendments comply with the requirements of 35 U.S.C. §112, first paragraph, and 35 U.S.C. §132(a). 

1. A combustion system comprising a combustion apparatus; a primary fuel feed line configured to feed a primary firing fuel to the combustion apparatus; a combustion air feed line configured to feed air to the combustion apparatus; a temperature controller configured to transmit a temperature demand signal based on a temperature in a combustion zone in the combustion apparatus; an analyzer controller configured to transmit a stoichiometric ratio control signal based on an excess oxygen level exiting the combustion zone in the combustion apparatus; and a cross-limiting override control system comprising: a dynamic stoichiometric ratio multiplier element configured to receive the stoichiometric ratio control signal transmitted from the analyzer controller; a high signal select element configured to receive the temperature demand signal transmitted from the temperature controller; and a low signal select element configured to receive the temperature demand signal transmitted from the temperature controller; wherein the cross-limiting override control system is configured to transmit remote set points to flow controllers configured to control the flow rates of the primary firing fuel and the air to the combustion apparatus.
 2. The combustion system according to claim 1, wherein the low signal select element is configured to transmit the temperature demand signal or a signal representing the air flow rate through the combustion air feed line, whichever signal has a lower numerical value, as a remote set point to a primary fuel flow controller.
 3. The combustion system according to claim 1, wherein the high signal select element is configured to transmit the temperature demand signal or a stoichiometrically-scaled fuel signal, whichever signal has a greater numerical value, as a remote set point to a combustion air flow controller.
 4. The combustion system according to claim 1, wherein the temperature controller is configured to transmit a temperature demand increase signal if the temperature in the combustion zone is less than a temperature set point, and wherein the temperature controller is configured to transmit a temperature demand decrease signal if the temperature in the combustion zone is greater than a temperature set point
 5. The combustion system according to claim 1, wherein the analyzer controller is configured to transmit a stoichiometric ratio increase signal if the excess oxygen level exiting the combustion zone is less than a predetermined set point, and wherein the analyzer controller is configured to transmit a stoichiometric ratio decrease signal if the excess oxygen level exiting the combustion zone is greater than a predetermined set point.
 6. The combustion system according to claim 1, wherein the combustion apparatus comprises a waste oxidizing boiler comprising at least one waste fuel feed line configured to feed a waste fuel to the combustion apparatus.
 7. The combustion system according to claim 6, wherein the cross-limiting override control system further comprises a summation element configured to receive signals representing the primary fuel flow rate through the primary fuel feed line and the waste fuel flow rate through the at least one waste fuel feed line, and transmit a feed forward signal to the dynamic stoichiometric ratio multiplier element representing the total fuel input to the combustion system.
 8. The combustion system according to claim 1, wherein the cross-limiting override control system is implemented as a logic control point on a distributed control system.
 9. A fuel management control process comprising: measuring a temperature in a combustion zone in a combustion apparatus; transmitting a temperature demand signal from a temperature controller based on the temperature in the combustion zone in the combustion apparatus; measuring an excess oxygen level exiting the combustion zone in the combustion apparatus; transmitting a stoichiometric ratio control signal from an analyzer controller based on the excess oxygen level exiting the combustion zone in the combustion apparatus; measuring a flow rate of a primary firing fuel to the combustion apparatus; transmitting a signal representing the flow rate of the primary firing fuel to the combustion apparatus; measuring a flow rate of air to the combustion apparatus; transmitting a signal representing the flow rate of the air to the combustion apparatus; receiving, at a logic control point, the temperature demand signal, the stoichiometric ratio control signal, the signal representing the flow rate of the primary firing fuel, and the signal representing the flow rate of the air; calculating, in the logic control point, remote set points for flow controllers configured to control the flow rates of the primary firing fuel and the air to the combustion apparatus; and transmitting the remote set points to the flow controllers.
 10. The fuel management control process according to claim 9, wherein the logic control point comprises: a dynamic stoichiometric ratio multiplier element receiving the stoichiometric ratio control signal and the signal representing the flow rate of the primary firing fuel to the combustion apparatus; a low signal select element receiving the temperature demand signal and the signal representing the flow rate of the air to the combustion apparatus; and a high signal select element receiving the temperature demand signal and a stoichiometrically-scaled fuel signal; wherein the dynamic stoichiometric ratio multiplier element multiplies the signal representing the flow rate of the primary firing fuel to the combustion apparatus by a stoichiometric ratio based on the stoichiometric ratio control signal to produce the stoichiometrically-scaled fuel signal, and wherein the dynamic stoichiometric ratio multiplier element transmits the stoichiometrically-scaled fuel signal to the high signal select element.
 11. The fuel management control process according to claim 10, further comprising: measuring a flow rate of a waste fuel to the combustion apparatus; transmitting to the logic control point a signal representing the flow rate of the waste fuel to the combustion apparatus; calculating a feed forward signal representing the total fuel input to the combustion system, wherein the feed forward signal is calculated from the signal representing the flow rate of the waste fuel to the combustion apparatus and the signal representing the flow rate of the primary firing fuel to the combustion apparatus; transmitting the feed forward signal to the a dynamic stoichiometric ratio multiplier element; and multiplying the feed forward signal by a stoichiometric ratio based on the stoichiometric ratio control signal to produce the stoichiometrically-scaled fuel signal.
 12. The fuel management control process according to claim 10, wherein the low signal select element transmits the temperature demand signal or the signal representing the air flow rate through the combustion air feed line, whichever signal has a lower numerical value, as a remote set point to a primary fuel flow controller.
 13. The fuel management control process according to claim 10, wherein the high signal select element transmits the temperature demand signal or the stoichiometrically-scaled fuel signal, whichever signal has a greater numerical value, as a remote set point to a combustion air flow controller.
 14. The fuel management control process according to claim 9, wherein the temperature controller transmits a temperature demand increase signal if the temperature in the combustion zone is less than a temperature set point, and wherein the temperature controller transmits a temperature demand decrease signal if the temperature in the combustion zone is greater than a temperature set point
 15. The fuel management control process according to claim 9, wherein the analyzer controller transmits a stoichiometric ratio increase signal if the excess oxygen level exiting the combustion zone is less than a predetermined set point, and wherein the analyzer controller transmits a stoichiometric ratio decrease signal if the excess oxygen level exiting the combustion zone is greater than a predetermined set point.
 16. The fuel management control process according to claim 9, wherein the cross logic control point is on a distributed control system. 